This invention relates generally to cellular wireless telephone systems and, in particular, to multi-frequency pilot beacons for hand-off of transceivers operating in Code Division Multiple Access (CDMA) systems.
A cellular communication system is one in which coverage is provided in relatively small areas, commonly referred to as cells, that overlap. These overlapping cells form a grid of radio coverage that extends over a region of interest, e.g., an urban area.
In traditional cellular systems each call or radio connection between a mobile transceiver (telephone) and a cellular base station occupies a narrow segment of the frequency spectrum allocated to the provider of the cellular service. Since each call must have its own frequency segment the total number of simultaneous calls which can be handled is limited by the number of segments in the frequency spectrum.
When the coverage area is broken up into cells, frequency segments or frequencies can be reused in cells that are far enough apart so that the signals at the same frequency do not interfere with one another. In a typical cellular system the frequency reuse factor (how many cells have to be operating on different frequencies before frequency reuse can occur) is 7. At this reuse factor cells reusing the same frequency are two cells away from each other. This also means, that only a seventh of the allocated frequency spectrum can be used within any given cell.
While moving within the cellular grid a mobile transceiver is forced to switch its operating frequency between the channels allocated to the different cells. This process is called xe2x80x9chand-offxe2x80x9d. In practice, the base station in one cell hands-off the transceiver call to a base station in another cell by forcing the transceiver to switch frequencies.
There are numerous problems with this traditional approach, often resulting in dropped calls and inefficient use of the frequency spectrum. Code Division Multiple Access (CDMA) technology is one of several alternative techniques for supporting cellular wireless communications in such a cellular system. CDMA systems have significant advantages over competing systems for multiple access communications such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and AM modulated systems such as Amplitude Companded Single Sideband (ACSSB) systems. Specifically, CDMA techniques result in a higher spectral efficiency than can be achieved using other multiple access techniques. In other words, more calls can be made in a given frequency band using CDMA than using other technologies.
In a prior art CDMA network 10 only one frequency band or carrier frequency, F1 is used by all cells 12, as shown in FIG. 1. A base station control 18, which operates base stations 20 in cells 12 does not issue frequency hand-off commands. That is because a transceiver 14 of a mobile user 16 does not have to hand-off between different frequencies in network 10.
Typically, a CDMA signal 22 in network 10 occupies a 1.25 MHz band (although other implementations can use more or less bandwidth). The band is centered at carrier frequency F1 and several CDMA signals are superimposed upon each other within the band. As shown in FIG. 2, each CDMA signal 22 is created by multiplying a narrow band (about 10 kHz wide) baseband signal 24 containing the data (e.g., voice data) by a spreading code which increases the resulting bandwidth to 1.25 MHz. On the forward link, from base station 20 to transceiver 14, CDMA signal 22 is prepared by spreading baseband signal 24 twice; once by a Walsh code and once by a pseudorandom noise sequence PN.
Baseband signal 24 is multiplied in a mixer 28 with a Walsh code Wi provided from a Walsh code generator 26 to produce a coded signal 30. Since individual Walsh codes are orthogonal their inner product satisfies the following condition:             W      i        *          W      j        =      {                            0                                      i            ≠            j                                                N                                      i            =            j                              
Thus, baseband signal 24 multiplied by Walsh code Wi on the forward link can only be demodulated by a receiver by multiplying it with the same Walsh code, i.e., Wi. Multiplication with any other Walsh code will not yield a signal. Hence, the receiver set to use Walsh code Wiwill reject all signals which are prepared with any Walsh code other than Wi.
Walsh coded signal 30 is then multiplied with the aid of mixers 32, 34 by a short pseudorandom noise sequence PN provided by a PN generator 36. The PN sequence has a characteristic offset. In this case coded signal 30 is multiplied by an in-phase and a quadrature portion of the PN sequence in accordance with standard modulation techniques. The multiplied signals are converted from digital to analog and filtered by circuits 25, 27 and then combined by a combining circuit 38. The thus created CDMA signal 22 is up-converted by a mixer 23 to carrier frequency F1 and sent to antenna 40 for transmission.
Since the same carrier frequency F1 is used throughout CDMA network 10 base stations 20 are assigned unique offsets of the PN sequence to distinguish them. For example, base station 20A uses sequence PNA which is the PN sequence with an offset A in generating its CDMA signals, base station 20B uses sequence PNB, and so on. The various sequences PNA, PNB, . . . etc. are generated by shifting the standard PN sequence by varying offset amounts also referred to as PN offsets. The PN sequences are used to multiply each channel including a pilot channel. The pilot channel is defined as the unmodulated Walsh code zero. In other words, the pilot channel requires that generator 26 be set to zero and baseband signal 24 be zero. As a result, only the PN sequence with its given PN offset is transmitted in the pilot channel.
Just as in the case of frequency reuse, PN sequences with the same offsets can be reused in cells 12 which are sufficiently far apart to avoid interference, e.g., cells 12A and 12B use the same sequence PNA. Transceiver 14 will examine the different PN offsets to thus identify base stations 20 near it. As user 16 moves from one cell 12 to another, transceiver 14 can hand off to a neighboring base station in a soft hand-off process. The carrier frequency remains the same but the PN sequence of the new base station is used. The process is called soft because during the transition from one base station to another transceiver 14 is communicating simultaneously with both base stations.
As the number of mobile telephone users increases, more capacity than offered by CDMA network 10 will be required. One way to accomplish this goal is to use more frequency channels within the allocated frequency spectrum by adapting CDMA network 10 to operate at more than one carrier frequency. This means that CDMA network 10 will have to accommodate hard or frequency hand-off between different frequencies used in different cells 12.
The prior art teaches the use of a pilot channel assigned Walsh code zero (0) to carry the PN offset information. The signal corresponding to the PN offset information is referred to as the pilot beacon. Knowledge of the PN offset allows the transceiver to identify with which base station they are communicating.
In U.S. Pat. No. 5,848,063 Weaver, Jr. et al. discusses the use of a pilot beacon for handing-off between dissimilar CDMA networks. The hand-off is not necessarily a frequency hand-off (hard hand-off) and the teaching is directed primarily at the hand-off algorithm and uses the measured time delay for the pilot beacon between the base station and the transceiver as a parameter for deciding when to execute a hand-off. U.S. Pat. No. 5,697,055 to Gilhousen et al. also discusses algorithms for determining hand-off between different cellular systems.
In U.S. Pat. No. 5,858,661 Weaver, Jr. et al. teach a method for creating areas where certain transceivers cannot communicate with certain base stations. These regions of silence are indicated by the presence of a pilot beacon with a specific PN offset indicating that any mobile transceiver hearing this pilot beacon is within the silence region.
In U.S. Pat. Nos. 5,267,261 and 5,101,501 Blakeney, II et al. and Gilhousen et al. teach the details of soft-hand off using pilot channels radiating pilot beacons. Each base station transmits a pilot beacon or pilot tone with a specific PN offset. All pilot beacons are transmitted at the same frequency.
In a CDMA system using various frequencies hand-off, in particular hard hand-off or frequency hand off between cells presents a new challenge. None of the prior art teaches how to produce a pilot beacon which can be used for executing such hand-offs in such CDMA networks.
Accordingly, it is a primary object of the present invention to provide a pilot beacon for use in a CDMA system using different carrier frequencies. Specifically, the object of the invention is to provide a multi-frequency pilot beacon for transmitting PN offset information to cellular users.
It is another object of the invention to adapt a CDMA system to use a multi-frequency pilot beacon for performing hard hand-off operations.
Yet another object of the invention is to provide a multi-frequency pilot beacon which is easy to manufacture and integrate in a CDMA system. The multi-frequency pilot beacon can be used in various configurations, including in-building micro-cells.
The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below.
These objects and advantages are secured by a multi-frequency pilot beacon adapted to a CDMA system using at least two different carrier frequencies, such as a first CDMA carrier frequency F1 and a second CDMA carrier frequency F2. The pilot beacon has a pseudorandom noise generator for supplying a pseudorandom noise sequence PN. It also has a frequency conversion mechanism for converting the PN sequence to a first pilot beacon centered at the first CDMA carrier F1 and a second pilot beacon centered at the second CDMA carrier F2. A transmitting unit transmits the first and second pilot beacons to the transceiver or mobile cellular unit.
The multi-frequency pilot beacon can be provided at a base station of a given cell to transmit the pilot beacons within that cell. The base station antenna can be used for transmitting the pilot beacons in this embodiment. Alternatively, multi-frequency pilot beacons can be provided wherever necessary within the CDMA system. In this situation the pilot beacons can be transmitted directly from the pilot beacon unit.
The PN sequences (in-phase and quadrature) are preferably digital sequences. The pilot beacon is equipped with a digital-to-analog converter for converting these digital PN sequences to an analog PN sequences.
In one embodiment the pilot beacon generates the PN sequences at an intermediate frequency (IF). Additional circuit elements are provided to accommodate this alternative.
A CDMA system using the multi-frequency pilot beacon uses the pilot beacons to hand-off the cellular transceiver between carrier frequencies. The carrier frequencies can be used in the same cell or in different, e.g., adjacent cells. Preferably, a hand-off order is issued by a CDMA system controller based on the traffic volume at the carrier frequencies involved. Alternatively, the hand-off order can be based on the location of the transceiver.
There are many methods for operating the multi-frequency pilot beacon. The pilot beacons can be transmitted together from the same location or from different locations or even from the base station. The configuration of the specific CDMA network can impose additional requirements on how to use the multi-frequency pilot beacon of the invention.